1. Field of the Invention
This invention relates to the treatment of seizure disorders and, and more particularly relates to techniques for warning patients of an epileptic seizure.
2. Description of Related Art
Epilepsy is a condition characterized by recurrent seizures which are the outward manifestation of excessive and/or hyper-synchronous abnormal electrical activity of neurons in the cerebral cortex of the brain. A seizure often occurs when the electrical activity of the brain becomes more "synchronized" as would be the case when the person is in a drowsy state.
A seizure patient may suffer from any combination of different types of seizures. Grand mal seizures are the most common form of epilepsy and are characterized by convulsions with tonic-clonic contractions of the muscles. Absence seizures (previously referred to as "petit mal") are characterized by a brief and sudden loss of consciousness. The psychomotor form of seizures is characterized by a clouding of consciousness for one or two minutes. A complex partial seizure is characterized by a complete loss of consciousness. The type of seizure experienced is typically dependent upon the function of the portion of the cerebral cortex where hypersynchronous activity is occurring. Many types of seizures generally involve the entire brain, while certain types, such as partial seizures, begin in one part of the brain and may remain local.
Regardless of the type of epilepsy, seizures significantly limit the autonomy of the patient. When hit with a seizure attack, the patient typically loses some level of control of his/her body. In most cases, seizures occur without prior warning to the patient. As a result, epileptic seizures pose a serious safety hazard to the patient as others surrounding the patient. For example, a patient hit with a sudden seizure attack while he/she is driving a car may endanger his/her own safety as well as the safety of others. Seizure patients are also exposed to a risk of bodily harm when operating machinery and even in daily activities such as crossing a street or going down stairs.
Researchers have developed a number of techniques for treating seizure disorders and its symptoms. For example, research has shown that inhibiting (namely, reducing the excitation of neurons) the substantia nigra in the brain increases the threshold for seizure occurrence. Researchers have also found that increasing the activity of neurons in the external Globus Pallidum (GPe) increases inhibition of neurons in the subthalamic nucleus which in turn inhibits neural activity in the substantia nigra. Neurosurgeons have also been able to diminish the symptoms of many neural disorders by lesioning certain brain areas, examples being lesioning the ventral lateral portion of the internal Globus Pallidus and the Vim Thalamus for treating movement disorders. Alternatively, it has been demonstrated that open-loop Deep Brain Stimulation (DBS) at high frequencies (100 Hz or higher) of certain brain structures can alleviate, diminish, or completely stop symptoms of tremor, rigidity, akinesia or hemiballism much like creating a lesion. Electrical stimulation of the nervous system has also been used to suppress seizures. Finally, infusion of certain drugs into a region of the brain can affect the excitability of the neurons at the site of infusion as disclosed in U.S. Pat. No. 5,713,923 (Rise et al.) assigned to Medtronic, Inc.
Others have studied the effects of electrically stimulating the vagus nerve as a means of "desynchronizing" the electrical activity of the brain. It has been observed that stimulation of the vagus nerve with certain parameters caused de-synchronization of the brain's electrical activity in animal models. These concepts were disclosed by Zabara in U.S. Pat. Nos. 4,867,164 and 5,025,807. De-synchronization can be thought of as "alerting" phenomena since it reflects active mental activity.
Under another approach, researchers have devised algorithms to detect the onset of a seizure. Qu and Gotman reported a system that recognizes patterns of electrical activity similar to a template developed from recording an actual seizure. See H. Qu and J. Gotman, "A Seizure Warning System for Long-term Epilepsy Monitoring", Neurology, 1995;45:2250-2254. Similarly, Osario et. al. have reported an algorithm applied to signals recorded from intracranial electrodes capable of 100% seizure detection rate with 0% false negatives and minimal false positives. See I. Osario, M. Frei, D. Lerner, S. Wilkinson, "A Method for Accurate Automated Real-time Seizure Detection", Epilepsia, Vol. 36, Suppl. 4, 1995. In each of these techniques for recognizing the onset of a seizure, the developers employ two processes. The first process is to extract certain features from the signals representing the electrical activity of the brain. Examples of the signal features include the signal power or the frequency spectrum of the signals. The second process is to recognize a pattern or set of values for those features which characterize a brain state that will reliably lead to a seizure.
Using these pattern recognition techniques, researchers have developed warning systems to alert the seizure patient of a possible seizure onset. For example, U.S. Pat. Nos. 3,863,625 and 4,566,464 disclose epileptic seizure warning systems producing audio and visual warning signals to the patient prior to the possible onset of a seizure. The warning devices are external devices which can be worn in a shirt pocket of a seizure patient. Such an approach is generally simpler in design than the above-described techniques for treatment of seizures and provides the patient prior warning of a possible seizure onset. Advantageously over the above-mentioned methods, these device allows the patient to take appropriate action to minimize physical injury to the patient and others. This approach, however, requires the patient to constantly carry around the monitoring device causing inconvenience to the patient especially if the monitoring device is misplaced. The use of audio and visual signals as the warning mechanism may also not be as effective if the device is buried underneath a heavy coat or is outside of the hearing range of the patient. In addition, this device is ineffective for seizure patients who have impaired vision or hearing.
The present invention is directed to overcoming the disadvantages of the foregoing systems.